Cataracts are a major cause of blindness in the world and the most prevalent ocular disease. When the disability from cataracts affects or alters an individual's activities of daily living, surgical lens removal with intraocular lens (“IOL”) implantation is the preferred method of treating the functional limitations.
A cataract is any opacity of a patient's lens, whether it is a localized opacity or a diffuse general loss of transparency. To be clinically significant, however, the cataract must cause a significant reduction in visual acuity or a functional impairment. A cataract occurs as a result of aging or secondary to hereditary factors, trauma, inflammation, metabolic or nutritional disorders, or radiation. Age related cataract conditions are the most common.
In treating a cataract, the surgeon removes the crystalline lens matrix from the lens capsule and replaces it with an IOL. The typical IOL provides a selected focal length that allows the patient to have fairly good distance vision. Since the lens can no longer accommodate, however, the patient typically needs glasses for reading.
More specifically, the imaging properties of the human eye are facilitated by several optical interfaces. A healthy youthful human eye has a total power of approximately 59 diopters, with the anterior surface of the cornea (e.g. the exterior surface, including the tear layer) providing about 48 diopters of power, while the posterior surface provides about −4 diopters. The crystalline lens, which is situated posterior of the pupil in a transparent elastic capsule, also referred to herein as “capsular sac,” supported by the ciliary muscles via zonules, provides about 15 diopters of power, and also performs the critical function of focusing images upon the retina. This focusing ability, referred to as “accommodation,” enables imaging of objects at various distances.
The power of the lens in a youthful eye can be adjusted from 15 diopters to about 29 diopters by adjusting the shape of the lens from a moderately convex shape to a highly convex shape. The mechanism generally accepted to cause this adjustment is that ciliary muscles supporting the capsule (and the lens contained therein) move between a relaxed state (corresponding to the moderately convex shape) and a contracted state (corresponding to the highly convex shape). Because the lens itself is composed of viscous, gelatinous transparent fibers, arranged in an “onion-like” layered structure, forces applied to the capsule by the ciliary muscles via the zonules cause the lens to change shape.
Isolated from the eye, the relaxed capsule and lens take on a more spherical shape. Within the eye, however, the capsule is connected around its circumference by approximately 70 tiny ligament fibers to the ciliary muscles, which in turn are attached to an inner surface of the eyeball. The ciliary muscles that support the lens and capsule therefore are believed to act in a sphincter-muscular mode. Accordingly, when the ciliary muscles are relaxed, the capsule and lens are pulled about the circumference to a larger diameter, thereby flattening the lens, whereas when the ciliary muscles are contracted the lens and capsule relax somewhat and assume a smaller diameter that approaches a more spherical shape.
As noted above, the youthful eye has approximately 14 diopters of accommodation. As a person ages, the lens hardens and becomes less elastic, so that by about age 45-50, accommodation is reduced to about 2 diopters. At a later age the lens may be considered to be non-accommodating, a condition known as “presbyopia”. Because the imaging distance is fixed, presbyopia typically entails the need for bi-focals to facilitate near and far vision.
Apart from age-related loss of accommodation ability, such loss is innate to the placement of IOLs for the treatment of cataracts. IOLs can be single element lenses made from a suitable polymer material, such as acrylics or silicones. After placement, accommodation is no longer possible, although this ability is typically already lost for persons receiving an IOL. There is significant need to provide for accommodation in IOL products so that IOL recipients will have accommodating ability. In addition, although efforts have been made with accommodating IOLs, there is a need for an accommodating IOL that can restore as much accommodation to the eye as possible.
What is needed is a device to test an accommodative intraocular lens to measure the intraocular lens's accommodative response to a force that is applied to it. It may also be desirable that the device be able to measure the intraocular lens's accommodative response to a simulated external actuation of the lens.